Certain materials, referred to as electrochromic materials, are known to change their optical properties in response to the application of an electrical potential. This property has been taken advantage of to produce electrochromic devices which can be controlled to transmit optical energy selectively.
A number of factors affect the operation of an electrochromic device. One limitation on how dark an electrochromic device can become is how much charge can be stored in the counter electrode layer. In this context, the term, “charge,” refers to the amount of electronic charge, or quantity of electrons per unit area, and the equivalent, corresponding quantity of charge balancing lithium ions per unit area, which may be conveniently expressed in units of milliCoulombs per square centimeter (mC/cm2). There have been many different approaches for producing a charge storage medium, but most attention has focused on a thin film deposited parallel to the electrochromic material layer, and separated by an ionically conductive layer.
To date, most counter electrode layers have been made using NiO, LiNiO, or doped variants thereof. One advantage of using NiO and LiNiO materials is that under careful preparation conditions, the counter electrode can be made so that it displays anodic electrochromism with good electrochromic efficiency and a high bleached state light transmission. Here, the term, “electrochromic efficiency” refers to the modulation of optical density per amount of charge transferred per unit area. Unfortunately, it has been difficult to intercalate lithium into NiO based materials as a result of the material's compact crystalline structure. As such, higher voltages must be applied to such materials to intercalate lithium, in order to drive the electrochromic response at a reasonably fast rate, which leads to undesirable side reactions.
Other methods employ protons, or hydrogen ions instead of lithium ions, as the charge balancing counter ion for the coloration mechanism. These methods may use counter electrode layers comprised of nickel hydroxides, or iridium oxides and other mixtures containing iridium. Typically an aqueous medium is also required to provide a suitable source of protons. Although it may be relatively easy to manufacture a counter electrode layer capable of coloring anodically in an aqueous medium, it is difficult to produce a complete device capable of long-term stability. It is, therefore, more advantageous to use lithium intercalation based systems.
A typical material used for counter electrode applications with lithium is vanadium oxide, which is a material that forms crystal structures similar to those seen in tungsten oxide systems. The open crystalline lattice of vanadium oxide allows lithium intercalation more readily than in NiO based structures. However, the presence of vanadium ions leads to the generation of a strong yellow color. This yellow color is only slightly modulated by lithium intercalation, and shows a reasonable cathodic electrochromic effect throughout the majority of the visible region, thus limiting the maximum transmission that can be achieved using this material as a counter electrode layer. Attempts to reduce the degree of coloration by doping vanadium oxides with other components result in a reduced electrochromic efficiency by reduction of the charge capacity of the counter electrode layer. Such doping results in a device with a higher bleached state transmission at the cost of decreased range of modulation of optical density.
There remains a need for improved electrochromic coatings, and in particular electrochromic coatings that comprise solid state, inorganic thin films, and metal oxide or metal oxide-containing thin films.
The foregoing examples of the related art and limitations related therewith are intended to be illustrative and not exclusive. Other limitations of the related art will become apparent to those of skill in the art upon a reading of the specification and a study of the drawings.